In brake systems with antilock control systems (ABS) or electronic stability programs (ESP), analogized valves (“AD valves”) are often used to control the hydraulic fluid. The current at which the AD valve opens is heavily dependent on the pressure difference across the valve. In order to be able to control the AD valves, it is necessary, for example, to know the value of this current, which is referred to as the “opening point”. The opening point is furthermore dependent on manufacturing tolerances and on the wear of the valve and of other components in the brake system.
In order to operate valves with sufficient accuracy in an analog manner, it is necessary to know the opening characteristic or, more generally, the control characteristic. To configure the production of the AD valves in such a way that each valve exhibits identical control characteristics (e.g. current/pressure characteristic curve), is not possible owing to tolerances or represents an unacceptable expense. Even if this were possible, change over the service life represents a further problem. Calibrating each valve individually is very time-consuming in production and increases costs considerably. However, without being able to operate the valves with appropriate accuracy, functional utility decreases.
A method for calibrating the pressure/opening current characteristic of an analogized or analog-controlled hydraulic inlet valve is known from DE 10 2006 057 501 A1, which is incorporated by reference. The method is carried out in a brake system for motor vehicles having two brake circuits, wherein each brake circuit is assigned two wheel brakes, a hydraulic pump and a low-pressure reservoir and wherein a brake pressure produced with the aid of the hydraulic pump passes via an inlet valve to a wheel brake and can be passed out of the wheel brake into a low-pressure reservoir via an outlet valve. In this case, a pressure sensor is provided, which is assigned to one of the two wheel brakes and determines the wheel brake pressure fed into said wheel brake. Thus, just one wheel brake sensor for each brake circuit is necessary to carry out this method.